It was impolite and circumspect of me; I should have said what was suggested above. I believe you have confused Challenge Levels with Challenge Classes.
Yes, that is what I meant. I am less concerned with the actual parity with HD, and more with the correct application of terms. Otherwise I don't know if CL=15 means they just wanted it to be impossible, or if it was a mistake.
If you do the +0/+6 thing it would work. If you do 12/18 as I do, then there would be no way to factor in the "primeness" of the check.
I may as well be plain about my concerns. This made it through at least one playtest group and an editor. C&C is often accused of having poor editing, sometimes fairly. My personal goodwill goes a long way towards tolerating this sort of thing, but it bothered me to think that the module writer might be unconcerned with the rule system. It's true that C&C CKs are a flexible bunch, and many use the books as a mere reference but really just run their own internalized (A/O)D&D. However, if a module is to receive the C&C stamp, and I am to pay money for it, I'd like it to follow the same system as all of the other [correctly edited] modules.
I hate writing this because I hate reading posts like this, but as a huge fan of C&C I hate to see the system become diluted, and I hate it when people on other boards have actual fuel for hating on it. And at the same time, I am not even sure my accusation is fair, because I have not looked at every CL across all of the "Yr Wyddfa" modules, and maybe they really are CLs (and I hope the author will say if this is so.)
I am a fan of the Welsh-inspired material and have no problems so far with it or its general presentation.
I think from now on, though, when I find mistakes in materials I buy from TLG, I am going to be loud about them; when I become silent it's because I moved onto something else, like the excellent Astonishing Sorcerers and Swordsmen of Hyperborea, which came out the gate with very professional editing, inspiring artwork, and the attentive support of Jeffrey Talanian.
My apologies for the rant.